Dr Stuart Marlin

1. Responses of complex cells in cat striate cortex were studied with flashed light slit stimuli. The responses to slits flashed in different positions in the receptive field were assessed quantitatively before and after periods of prolonged stimulation of one small region of the receptive field. This type of prolonged stimulation resulted in reduced responsivity over a limited zone within the complex cell receptive field. 2. The adaptation-induced responsivity decrement was generally observed in both the ON and OFF response profiles but could also be restricted to one or the other. In general, the magnitude of the response decrements was greatest in the ON response profiles. The adaptation-induced response decrement did not necessarily spread throughout the receptive field but was restricted to a small region surrounding the adapted receptive field position (RFP). Adaptation spread equally widely across the ON and OFF response profiles despite the smaller adaptation effects in the OFF profile. 3. The adaptation effects from repeated stimulation at a single RFP did not spread symmetrically across the receptive field, and a given cell's preferred direction of motion indicated the direction of the asymmetric spread of the adaptation. RFPs that would be stimulated by a light slit originating at the point of adaptation and moving in the preferred direction (preferred side) showed greater adaptation-induced response decrements than did RFPs that would be stimulated by a light slit moving in the opposite direction from the point of adaptation (nonpreferred side). There was significant enhancement of responses at some RFPs on the nonpreferred side of the point of adaptation. This asymmetric spread of adaptation could be caused by adaptation of inhibitory connections that contribute to complex cell direction selectivity. 4. The asymmetry of adaptation was significantly different for the ON and OFF response profiles. The asymmetric spread of adaptation for the ON response profile was similar to that observed previously in simple cells with greater decrements in the preferred direction side of the point of adaptation. However, the OFF response profiles showed less directional asymmetry in the spread of adaptation and showed greater decrements at RFPs in the nonpreferred direction side of the point of adaptation. 5. The similarity between the spread of adaptation in simple and complex cells suggests that the adaptation in these cells is occurring through a common mechanism. The directional asymmetry of the spread of adaptation is likely due to a local postsynaptic mechanism of adaptation rather than presynaptic transmitter depletion.

1. Responses of single cortical neurons in area 17 of anesthetized cats were recorded in response to prolonged stimulation with a patch of drifting square-wave grating. 2. During adaptation in the preferred direction, all neurons showed some reduction in response to motion in the stimulated direction and most showed some reduction in the opposite, nonstimulated direction. 3. For complex cells, the time course of response decrement in both the stimulated and nonstimulated directions was exponential, with an average time constant of 5 s. Response recovery was also exponential but significantly slower, with time constants of 8 and 13 s in the stimulated and nonstimulated directions, respectively. 4. For simple cells the dynamics of the adaptation effect depended on the direction of testing. In the nonstimulated direction the time course of the change in sensitivity was similar to that of complex cells. In the stimulated direction during both the adaptation and recovery periods, simple cells showed an initial rapid exponential change on the order of a few seconds that was followed by a more gradual exponential change. 5. During prolonged stimulation in the nonpreferred direction, there was less overall change in sensitivity. For some neurons the change in sensitivity during adaptation and recovery was exponential, with a short time constant for both simple and complex cells and for stimulated and nonstimulated directions. Other neurons showed no change in sensitivity in either direction and a few neurons showed facilitation during the adaptation period. 6. There appears to be a rapid general or nonspecific process, which may be related to contrast gain control, underlying motion adaptation in striate cortical neurons. An additional slow, direction-selective process is revealed when simple but not complex cells are stimulated in the preferred direction. We suggest that this latter type of adaptation is a key feature underlying the perceptual motion aftereffect.

1. Responses of simple cells in cat striate cortex were studied with flashed light-slit stimuli. The responses to bars flashed in different positions in the receptive field were assessed quantitatively before and after periods of prolonged stimulation of one small region. This type of prolonged stimulation resulted in reduced responsivity over a limited zone within the simple cell receptive field. 2. The adaptation-induced responsivity decrement was generally confined to the receptive-field subregion that was adapted (either ON or OFF). Prolonged stimulation within an ON region did not usually result in adaptation effects that spread into neighboring OFF regions. Furthermore, the adaptation-induced response decrement did not necessarily spread throughout the subregion in which the adapting stimulus was presented. The adaptation effects from prolonged stimulation at a single receptive-field position spread throughout the subregion in nearly one-half of the 25 cells examined for position-specific adaptation. Another subpopulation of neurons (n = 12) displayed adaptation effects that spread through only one-half of the subregion, whereas in two neurons the spread of the adaptation effect was even more restricted and encompassed only one-fourth of the subregion. 3. The spread of adaptation was not systematically related to the size of the stimulus presented, the size of the receptive field, or the magnitude of the adaptation-induced response decrements but was significantly correlated with the spatial wavelength of the cell (the reciprocal of the cell's preferred spatial frequency) and with the size of the subregion in which the adapting stimulus was presented. Cells with large receptive-field subregions and long wavelengths showed adaptation effects that spread further than those of cells with small subregions. 4. The adaptation effects from repeated stimulation at a single receptive-field position did not spread symmetrically across the receptive field, and the preferred direction of motion for a given cell indicated the direction of the asymmetric spread of the adaptation. Receptive-field positions that would be stimulated by a light slit originating at the point of adaptation and moving in the preferred direction (preferred side) showed greater adaptation-induced response decrements than did receptive-field positions that would be stimulated by a light slit moving in the opposite direction from the point of adaptation (nonpreferred side). There was significant enhancement of responses at some receptive-field positions on the nonpreferred side of the point of adaptation. The greatest degree of directional asymmetry occurred at ~0.20¿ from the point of adaptation (where ¿ is the cell's spatial wavelength), a finding that has strong implications for models of direction selectivity. This asymmetrical spread of adaptation could be caused by inhibitory connections that contribute to simple cell direction selectivity. 5. These adaptation effects displayed strong interocular transfer in binocularly driven simple cells. The locus of adaptation is, therefore, likely to be in the cortex.

The selectivity of adaptation to unidirectional motion was examined in neurons of the cat striate cortex. Following prolonged stimulation with a unidirectional high-contrast grating, the responsivity of cortical neurons was reduced. In many units this decrease was restricted to the direction of prior stimulation. This selective adaptation produced changes in the degree of direction selectivity of the cortical units (as measured by the ratio of the response to motion in the preferred direction to that in the nonpreferred direction). The initial strength of the directional preference of a given cortical unit did not determine the degree of direction-selective adaptation. Indeed, even non-direction-selective units could exhibit pronounced direction-selective adaptation. The degree of direction-selective adaptation was also independent of the overall decrease in responsivity during adaptation. There was no difference between simple and complex cells in the total amount of adaptation observed. The selectivity of the adaptation, however, did differ between these two cell types. As a group, simple cells showed significant direction-selective adaptation, whereas complex cells did not. The directional preference of most simple cells decreased following preferred direction adaptation and many highly direction selective simple cells became non-direction selective. In addition, simple cells became significantly more direction selective following nonpreferred direction adaptation. Some complex cells also demonstrated direction-selective adaptation. There was, however, much more variability among complex cells than simple cells. Some complex cells actually increased direction selectivity following preferred direction adaptation. These differences between simple and complex cells suggest that changes in direction selectivity following unidirectional adaptation are not due to simple neuronal fatigue of the unit being recorded, but depend on selective adaptation of afferent inputs to the unit. The spontaneous activity of many cortical neurons decreased following preferred direction adaptation but increased following adaptation in the nonpreferrd direction. The response to a stationary grating also decreased following preferred direction adaptation. However, there was very little change in the response to a stationary grating following adaptation in the nonpreferred direction.

Several assays were used in assessing conditioned inhibition within a taste aversion procedure. Following Pavlovian conditioned inhibition training, in which one taste was followed by an injection of LiCl on A+ trials, but was followed by access to a second flavored solution on AX- trials, retardation-of-acquisition and summation tests failed to indicate that the X stimulus (NaCl) had become inhibitory. Nor was the X stimulus consistently preferred to water or dilute quinine in two-bottle tests, contrary to an earlier report (Best, 1975). © 1986 Psychonomic Society, Inc.